This work discusses optical fiber sensors based on lossy-mode resonance (LMR) effect and their potential for simultaneous sensing in multiple domains, i.e., optical and electrochemical. As electrically conductive materials able to guide lossy modes, two doped tin oxides, i.e., fluorine doped tin oxide (FTO) and indium tin oxide (ITO) thin films were employed. Since the ITO-LMR sensor has already been discussed broader, this work focuses on properties of the FTO-LMR sensor and brief comparison of devices based on the two materials. In optical domain the sensitivity to surrounding medium refractive index was determined by immersing the sensors in solutions of different refractive index. Both the sensors showed sensitivity of 300 nm/RIU in a refractive index range of approx. 1.33-1.39 RIU. Electrochemical measurements were performed in 0.01 M phosphate-buffered saline (PBS, pH 7.0) to identify the influence of the applied potential on the optical response of both sensors. In applied potential from -1.0 V to 1.0 V the FTO-LMR sensor reached LMR shift of 31.3 nm compared to 23.8 nm of the ITO-LMR one.
Glass and fiber structures with Indium Tin Oxide (ITO) coating were subjected to annealing in order to identify impact of the thermal treatment on their optical and electrochemical properties. It is shown that the annealing process significantly modifies optical properties and thickness of the films, which are crucial for performance of optical fiber sensors. Moreover, it visibly improves electrochemical activity of ITO on glass slides and thicker (∅=400 μm) ITO-coated fibers, whereas in the case of thinner fibers (∅=125 μm) it could lead to a loss of their electrochemical activity. Depending on the applied substrate and the annealing process, the investigated structures with ITO coating can be further used as fiber-based sensors with integrated opto-electrochemical readout.
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